Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so the safety profile is covered. The description adds the result ordering and a prompt-injection caution about untrusted sender content, which is useful context beyond annotations, though that caution sits oddly on a tool that lists mailboxes rather than message bodies.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The purpose is front-loaded in a short first sentence, which is good. The second sentence is a long security boilerplate block that spends most of the description's length on a concern tangential to listing mailboxes, so the balance of space is off.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With an output schema present and full parameter documentation, the description need not explain return values or parameters. What remains missing is the routing context: how this list relates to mailbox threads, message search, and tenant scoping, which is the main gap for a low-complexity list tool.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so limit, cursor and tenant are already documented in the schema, and the description adds no syntax, defaults, or format detail beyond it. Baseline 3 applies since the schema carries the semantics.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource plus an ordering guarantee ("List mailboxes, newest first"), which is more than the title alone. It does not name or distinguish itself from near-neighbors such as list_mailbox_threads, search_mailbox_messages, or list_tenants, so sibling differentiation is left to inference.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

There is no when-to-use or when-not-to-use guidance and no reference to alternatives, despite many closely related siblings (search_mailbox_messages, list_mailbox_threads, get_mailbox_message). A caller must guess whether this tool or search_mailbox_messages is appropriate for its task.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.